Preheat the oven to 350°F (180°C). Butter a 9×13 (23x33cm) pan, and line with parchment paper if desired.
Layer 6 phyllo sheets in the pan, one at a time. Brush each sheet generously with melted butter.
Let edges hang over the sides (this helps wrap the top later).
Pour the custard evenly over the phyllo base and smooth it out.
Fold over the overhanging phyllo sheets.
Add remaining 6 phyllo sheets on top, one at a time, buttering each layer.
Tuck the overhanging phyllo edges down along the sides of the pan.
Score the top lightly into 12 servings. This step is optional, but it helps with slicing later.
Bake for 35-40 minutes, until deeply golden and crisp.
Let it cool slightly, for about 20–30 minutes.
Dust generously with powdered sugar and cinnamon, if desired.
Slice and serve warm.